Traction control for a personal transporter

ABSTRACT

A device and method for maintaining traction between wheels of a transporter and an underlying surface. The method has the steps of comparing acceleration of a wheel with a specified pre-set value, setting a slip condition flag based on the acceleration of the wheel, incrementally reducing the torque applied to the wheel based on the slip condition flag, determining a value of a dynamic characteristic of the wheel such as the inverse moment of inertia, and clearing the slip condition flag based on the value of that dynamic characteristic of the wheel.

FIELD OF THE INVENTION

The present application is directed to modes of control for a personaltransporter utilizing an electrical power source.

BACKGROUND OF THE INVENTION

Dynamically stabilized transporters refer to personal vehicles having acontrol system that actively maintains the stability of the transporterwhile the transporter is operating. The control system maintains thestability of the transporter by continuously sensing the orientation ofthe transporter, determining the corrective action to maintainstability, and commanding the wheel motors to make the correctiveaction. If the transporter loses the ability to maintain stability, suchas through the failure of a component, the rider may experiencediscomfort at the sudden loss of balance. For some dynamicallystabilized transporters, such as those described in U.S. Pat. No.5,701,965, which may include a wheelchair for transporting a disabledindividual down a flight of stairs, it is essential, for the safety ofthe operator, that the vehicle continue to operate indefinitely afterdetection of a failed component. For other dynamically stabilizedtransporters, however, the operator may readily be capable of safelydismounting from the transporter in case of component failure. It isdesirable that control modes be provided for such vehicles from whichthe operator is capable of safely dismounting in case of mishap.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the present invention, thereis provided a method for maintaining traction between wheels of atransporter and an underlying surface. The method has the steps of:

(a) comparing acceleration of a wheel with a specified pre-set value;

(b) setting a slip condition flag based on the acceleration of thewheel;

(c) reducing the torque applied to the wheel based on the slip conditionflag;

(d) determining a value of a dynamic characteristic of the wheel; and

(e) clearing the slip condition flag based on the value of the dynamiccharacteristic of the wheel.

In accordance with alternate embodiments of the invention, the dynamiccharacteristic may be a moment of inertia or an inverse of a moment ofinertia. The step of determining the dynamic characteristic may includedividing the acceleration by a commanded torque applied to the wheel.The step of reducing the torque applied to the wheel may includereducing the torque to zero, as well as slewing the torqueincrementally.

In accordance with yet further embodiments of the invention, a device isprovided for correcting wheel slippage on a vehicle. The device has asensor for monitoring wheel speed, a differentiator for calculatingwheel acceleration based on change in wheel speed, a comparator forcomparing the wheel acceleration with a pre-set value and for setting aslip condition flag, and a controller for reducing any torque applied tothe wheel, such that torque continues to be reduced until the slipcondition flag is cleared.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a personal vehicle lacking a stable staticposition, for supporting or conveying a subject who remains in astanding position thereon;

FIG. 2 shows a block diagram of the system architecture of an embodimentof the present invention;

FIG. 3 shows a top view of the power source with the top cover removed;

FIG. 4 is a block diagram of the power drive module of an embodiment ofthe present invention;

FIG. 5 is an electrical model of a motor;

FIG. 6a shows a top view of a rider detector in accordance with anembodiment of the present invention;

FIG. 6b shows a cut side view of the embodiment of FIG. 6a;

FIG. 7 shows an exploded view of a yaw input device in accordance withan embodiment of the present invention;

FIG. 8a is a cross-sectional top view of an elastomer-damped yaw inputdevice, shown in its relaxed position, in accordance with an embodimentof the present invention;

FIG. 8b is a cross-sectional top view of the yaw input de vice of FIG.8a shown in a deflected position;

FIGS. 8c and 8 d are back and top views, respectively, of the yaw inputdevice of FIG. 8a coupled to a handlebar of a personal transporter inaccordance with an embodiment of the present invention;

FIGS. 9a and 9 b depict a palm steering device, in a rest state andactivated state, respectively, as implemented in a handlebar of apersonal transporter in accordance with an embodiment of the presentinvention;

FIG. 10 is a logical flow diagram of the control program in accordancewith embodiments of the present invention;

FIG. 11 is a flow diagram for traction control in accordance with anembodiment of the present invention; and

FIG. 12 is a flow diagram for deceleration-to-zero in accordance for anembodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A personal transporter may be said to act as ‘balancing’ if it iscapable of operation on one or more wheels but would be unable to standon the wheels but for operation of a control loop governing operation ofthe wheels. A balancing personal transporter lacks static stability butis dynamically balanced. The wheels, or other ground-contactingelements, that provide contact between such a personal transporter andthe ground or other underlying surface, and minimally support thetransporter with respect to tipping during routine operation, arereferred to herein as ‘primary ground-contacting elements.’

An embodiment of a balancing personal transporter in accordance with thepresent invention is depicted in FIG. 1 and designated generally bynumeral 10. In certain applications, operation of personal transporter10 may not require operation for an extended period of time in case offailure. Fail-operative operation may be desirable, however, for adefinite period of time in order to allow the transporter to maintainstability while stopping and permitting a user to alight from thevehicle. While certain balancing personal transporters may not berequired to operate indefinitely if a component fails, it may, however,advantageously provide fail-detect redundant architecture wherein thecritical components such as gyros, batteries, motor windings, andprocessors are replicated and run in parallel during operation of thetransporter. If a failure occurs in one line of components, the parallelline will still maintain the stability of the transporter for at least ashort period of time. In accordance with the present invention and asdiscussed below, the short period of continued operation isadvantageously used to bring the transporter to a stop while maintainingbalance and then turn off the wheel motors. The transporter is broughtto a stop by commanding the transporter to pitch backward as is done inspeed limiting.

User 8 is shown in FIG. 1, standing on platform (or ‘base’) 12 ofground-contacting module 26. Wheels 21 and 22 are shown as coaxial aboutthe Y axis. Steering or other control may be provided by thumbwheels 32and 34, or by other user input mechanisms described in detail below. Ahandlebar 14 may be provided on stalk 16 for gripping by the user.

Referring now to FIG. 2, a block diagram is shown of the systemarchitecture of an embodiment of the present invention. A left motor 110drives a left wheel 20 (shown in FIG. 1) and a right motor 120 drives aright wheel 21. Motors 110 and 120 are preferably DC brushless but maybe either AC or DC motors and either brushed or brushless. Each motor isenergized by a redundant set of windings 111, 112, 121, 122. Eachwinding is capable of energizing the motor in the event thecomplimentary winding is unable to energize the motor. In the discussionbelow, each redundant component is distinguished by a two letter groupidentifying either the left (L) or right (R) side of the transporter andeither the A group or B group of redundant components. For example, theleft motor winding energized by the A group of components is designatedas the LA winding.

Each of motor windings 111, 112, 121, 122 is driven by a motor amplifier132, 133, 142, 143. The A-group amplifiers 132, 133 are supplied by theA-group power supply 131 and the B-group amplifiers 142, 143 aresupplied by the B-group power supply 141. The electrical connectionsbetween the power supplies and amplifiers and between the amplifiers andmotor windings are expected to carry large currents up to 20 to 40Amperes and are identified by thick lines 105 in FIG. 2.

Each motor 110, 120 has a shaft feedback device (SFD) 113, 123 thatmeasures the position or angular velocity of the motor shaft. The SFD isin signal communication with the motor amplifiers driving the motorassociated with the SFD. For example, the right SFD 123 associated withthe right motor 120 is in signal communication with the RA amplifier 133and the RB amplifier 143. The SFD is preferably a Hall sensor thatdetermines the position of the shaft, however the SFD may be selectedfrom a variety of sensors such as encoders, resolvers, and tachometers,all listed without limitation for purposes of example. Certain sensors,such as tachometers, may also be used to measure the shaft velocity.Conversion of a signal representing instantaneous shaft velocity to orfrom a signal representing position is accomplished by integrating ordifferentiating the signal, respectively.

The A-group amplifiers 132, 133 are commanded by the A processor 135while the B-group amplifiers 142,143 are commanded by the B processor145. Power is supplied to the A processor from the A power source 131through the A-group DC-DC converter 136. Similarly, the B power source141 supplies power to the B processor 146 through the B-group DC-DCconverter 145. The A-group amplifiers 132, 133, A-group converter 136,and A processor 135 are preferably grouped together into a compartmentor tray 130 that is at least partially isolated by a barrier 150 fromthe B-tray 140 containing the B-group amplifiers, B-group converter, andB processor. Physically separating the A tray 130 and B tray 140 reducesthe probability of a common point failure. The barrier 150 acts to delaythe propagation of a failure, in one tray to the other tray such thatthe transporter has sufficient time to put the rider in a safe conditionto exit the transporter. Similarly, the A power supply 131 is physicallyseparated from the B power supply 141. The A power supply 131 and thecomponents in the A tray 130 are capable of driving both motors 110, 120for a short period of time, on the order of a few seconds, in the eventof a failure in any one of the B-group components. Conversely, the Bpower supply 141 and the components in the B tray 140 are capable ofdriving both motors 110, 120 for a short period of time if an A-groupcomponent fails.

Although the processors 135, 145 are physically isolated from eachother, signal communication is maintained between the processors viacommunication channels 137,147. Communication channels 137,147 arepreferably electrical conductors but may also be electromagnetic such asoptical, infrared, microwave, or radio. The A channel 137 transmitssignals from the A processor 135 to the B processor 145 and the Bchannel 147 transmits signals from the B processor 145 to the Aprocessor 135. Optical isolators 139, 149 are incorporated into channels137, 147 to prevent over-voltages from propagating from a shortedprocessor to the other processor.

Each processor receives signals from a plurality of sensors that monitorthe state of the transporter and the input commands of the rider. Theprocessor uses the sensor signals to determine and transmit theappropriate command to the motor amplifiers. The information transmittedto the processors by the sensors include the spatial orientation of thetransporter provided by an inertial measurement unit (IMU) 181, 182, therider directed turn command provided by a yaw input device (YID) 132,142, and the presence of a rider on the transporter provided by a riderdetector (RD) 161, 162, 163, 164. Other inputs to the processor mayinclude a rider operated pitch trim device (PTD) 148 for adjusting thepitch of the transporter to a more comfortable pitch and a stop button(not shown) for bringing the transporter to a stop quickly. Depending onthe importance of the sensor to the operation of the transporter, thesensors may or may not be duplicated for redundancy. For example, thespatial orientation of the transporter is central to the operation ofthe transporter, as is described below, and therefore an A-group IMU 181supplies transporter orientation information to the A processor 135 anda B-group IMU 182 supplies transporter orientation information to theB-processor 145. On the other hand, the transporter may still beoperated in a safe manner without the PTD 148 so only one such device istypically provided. Similarly, an output device such as a display 138does not require redundancy. A non-redundant device such as a display138 or a PTD 148 may be connected to either processor.

In the embodiment depicted in FIG. 2, display 138 is controlled by the Aprocessor 136 and the PTD 148 is in direct signal communication with theB processor 145. The information provided by the PTD 148 is transmittedby the B processor 145 to the A processor 135 via the B channel 147.

Additionally, each processor 135, 145 communicates with one of the userinterface processors (UIPs) 173, 174. Each UIP 173, 174 receivessteering commands from the user through one of the yaw input devices171,172. A A-group UIP 173 also communicates to the non-redundant UIDssuch as the display 138, brake switch 175, and pitch trim control 148.Other user interface devices that are not provided redundantly in theembodiment shown in FIG. 2, such as a sound warning device, lights, andan on/off switch, may also be connected to the A-group UIP 173. TheA-group UIP 173 may also pass along information provided by the userinterface devices to the B-group UIP 174.

In accordance with preferred embodiments of the invention, the A-groupUIP 173 compares calculations of the A-group processor with calculationsof the B-group processor and queries the A-group processor 135 with a‘watchdog’ calculation to verify operation of the A-group processor.Similarly, the B-group UIP 174 queries the B-group processor 145 toverify normal operation of the B-group processor.

Several components of personal transporter 10, in accordance withvarious embodiments of the present invention, are now described.

Battery

The transporter power required to drive the motors 110, 120 andelectrical components may be supplied by any known source of electricalpower known in the electrical arts. Sources of power may include, forexample, both internal and external combustion engines, fuel cells, andrechargeable batteries. In preferred embodiments of the presentinvention, power supplies 131, 141 are rechargeable battery packs.Various battery chemistry modalities may be used, as preferred undervarious conditions, and may include, without limitation, lead-acid,Lithium-ion, Nickel-Cadmium (Ni—Cd), or Nickel-metal hydride (Ni—MH)chemistry. Each power supply 131, 141 is enclosed in a container thatprotects the battery packs and associated electronics from theenvironment.

FIG. 3 shows a top view of one embodiment of the power supply with thetop cover removed. A tray 205 that is covered and sealed to protect thecontents from the environment encloses the components of power supply200. Tray 205 houses a plurality of battery blocks 210, each of whichcontains a plurality of battery cells 215. The number of cells 215packaged in a block 210 and the total number of blocks in the powersupply are determined by the expected power requirements of thetransporter. In a preferred embodiment, cells 215 are “sub-C”-size cellsand each block 210 contains ten cells 215. In another embodiments, block210 may contains other numbers of cells 215. Cells 215 are preferablyconnected in series, as are blocks 210. In other embodiments blocks 210may be connected in parallel with the cells 215 within each blockconnected in series, or, alternatively, blocks 210 may be connected inseries with the cells 215 within each block 210 connected in parallel,each configuration providing advantages for particular applications.

Electrical current flowing into or out of power supply 200 is conductedthrough a connector 220 that provides the electrical interface betweenthe power supply 200 and the transporter 10. In an embodiment shown inFIG. 3, connector 220 is located on the top cover (not shown) of powersupply 200 but any positioning of connector 220 is within the scope ofthe present invention. In addition to conducting current into or out ofpower supply 200, connector 220 may also include a plurality of signallines that establish signal communication between the power supplyinternals and any other transporter processor.

The temperature of each block 210 is monitored by the supply controller230 through temperature sensors 235. In addition, supply controller 230also monitors the voltage of each block 210. If supply controller 230detects that the temperature of a block 210 is over a preset temperaturelimit, the supply controller 230 sends an over-temperature signal to theprocessor through connector 220. Similarly, if supply controller 230detects that the voltage of a block 210 is below a preset voltage limit,the supply controller 230 sends an under-voltage signal to the processorthrough the connector 220.

Supply controller 230 preferably contains an ID chip 240 that storesinformation about the power supply such as battery type, the number ofcells in the power supply 210, and optionally, a date code or serialnumber code. The ID chip 240 may be of any type of permanent orsemi-permanent memory devices known in the electronics art. Theinformation contained in the ID chip 240 may be used by the processor135, 145 to set various operating parameters of the transporter. Theinformation may also be used by a charger (not shown) to recharge thepower supply.

Power supply 200 may be connected via connector 220 to a charger that iseither external to the transporter or contained within the transporter.In one embodiment of the present invention, the charger is located onthe transporter and is an AC switch mode charger well known in the powerart. In another embodiment, the charger is contained within battery tray205. In another embodiment of the present invention, power supply 200 ischarged by an auxiliary power unit (APU) such as the one described incopending U.S. patent application, Ser. No. 09/517,808 entitled“Auxiliary Power Unit”.

Motor Amplifier & Operating Modes

FIG. 4 shows a block schematic of a power module 300 of one embodimentof the present invention. A balancing processor 310 generates a commandsignal to motor amplifier 320 that, in turn, applies the appropriatepower to motor 330. Balancing processor 310 receives inputs from theuser and system sensors and applies a control law, as discussed indetail below, to maintain balance and to govern motion of thetransporter in accordance with user commands. Motor 330, in turn,rotates a shaft 332 that supplies a torque, τ, at an angular velocity,ω, to a wheel 20, 21 (shown in FIG. 1) that is attached to shaft 332. Insome embodiments, a transmission, not shown, may be used to scale thewheel speed in relation to the angular velocity of the shaft 332. In apreferred embodiment of the present invention, motor 330 is a three-coilbrushless DC motor. In that embodiment, motor 330 has three sets ofstator coils although any number of coils may be used. The stator coilsare electrically connected to a power stage 324 by coil leads 337capable of conducting large currents or high voltages. It is understoodthat the large currents and high voltages are relative to the currentsand voltages normally used in signal processing and cover the rangeabove 1 ampere or 12 volts, respectively.

Motor amplifier 320 itself contains both an amplifier processor 322 anda power amplification stage 324. Amplifier controller 322 may beconfigured to control either current or voltage applied to the motor330. These control modes may be referred to as current control mode andvoltage control mode, respectively. Power stage 324 switches the powersource 340 into or out of connection with each coil, with the switchingof the power stage 324 controlled by the amplifier controller 322. Aninner loop 326 senses whether the output of power stage 324 is ascommanded and feeds back an error signal to amplifier controller 322 ata closed loop bandwidth, preferably on the order of 500 Hz.Additionally, control by amplifier controller 322 is based, in part, ona feedback signal from shaft feedback sensor (SFS) 335.

Shaft feedback sensor 335 is also in signal communication with theprocessor 310 and provides information related to the shaft position ormotion to the processor. The shaft feedback sensor 335 may be any sensorknown in the sensor art capable of sensing the angular position orvelocity of a rotating shaft and includes tachometers, encoders, andresolvers. In a preferred embodiment, a Hall sensor is used to sense theposition of the rotating shaft 332. An advantage of a Hall sensor is thelow cost of the sensor. In order to obtain a measure of shaft rotationvelocity from a position signal provided by shaft feedback sensor 335,the position signal is differentiated by differentiator 308. The outerfeedback loop 342 operates at a bandwidth characteristic of the balancecontrol provided by balance processor 310 and may be as low as 20-30 Hz.

While current and voltage may be equivalent in certain applications,voltage control is advantageously applied in embodiments of transportercontrol where the outer loop bandwidth is more than 3-4 times slowerthan the inner closed loop bandwidth, for the reasons now discussed withreference to FIG. 5. FIG. 5 shows an electrical model 410 of a motor. Amotor has a pair of terminals 411, 412 across which a voltage V isapplied. Motor 410 also has a rotating shaft 420 characterized by ashaft velocity, ω, and a torque, τ. Motor 410 may be modeled by resistor430 of resistance R carrying a current i in series with an ideal motor435 having a voltage drop V_(errd). For an ideal motor, V_(emf),=k_(v)·ωand τ=k_(c) ·i where k_(v) and k_(c) are motor constants. Seriesresistor 430 models the losses of the motor 410.

The differences in behavior of transporter 10 (shown in FIG. 1) due tovoltage control or current control can be seen using the example of atransporter encountering and driving over an obstacle. When a wheel 20of the transporter encounters an obstacle, the wheel velocity willdecrease because the torque applied to the wheel is insufficient todrive the wheel over the obstacle. The drop in wheel velocity will bereflected in a decrease in the back-electromotive-force (“back-emf”)voltage across the ideal motor.

Considering, first, the case of voltage control: If the amplifier is involtage control mode, the voltage applied to terminals 411, 412 remainsconstant and additional current will be drawn through resistance 430 andideal motor 435. The additional current through the motor will generatethe additional torque to drive the wheel over the obstacle. As thetransporter drives over the top of the obstacle, the wheel willaccelerate under the additional torque that was generated to drive overthe obstacle but is no longer required to drive off the obstacle. As thewheel accelerates, the back-emf across the motor will increase and thecurrent through R will decrease in order to keep the voltage acrossterminals 411, 412 constant. The decrease in current reduces the appliedtorque generated by the ideal motor thereby reducing the acceleration ofthe wheel. The advantage of voltage control mode is that the ideal motornaturally draws the current required to drive over the obstacle andnaturally reduces the current to drive off the obstacle without anychange required in the motor command. As long as the power source cansupply the required current, the motor essentially acts as its ownfeedback sensor and the control loop delay for the motor is essentiallyzero.

Under current control mode, on the other hand, the amplifier will keepthe current constant through resistor 430 and ideal motor 435 until thecontroller sends a new current command during the next processor frame.When the wheel encounters the obstacle, ω decreases and the back-emfacross the ideal motor decreases. However, since the amplifiercontroller is keeping the current constant, the voltage across terminals411, 412 is allowed to drop. Since the current is held constant by theamplifier controller, the torque remains constant. However, the torqueis insufficient to drive over the obstacle and the inertia of the movingtransporter will cause the transporter to pitch forward. As thetransporter begins to pitch forward over the obstacle, the balancingcontroller will detect the pitching, either through a change in thepitch error or through a change in the velocity, and command an increasein current to the amplifier controller, in accordance with the controlalgorithm taught in U.S. Pat. No. 5,971,091. The motor amplifier willrespond to the increased current command by supplying additional currentthrough R and the ideal motor. The increased current through the idealmotor increases the torque applied to the wheel until it is sufficientto drive the wheel over the obstacle. As the transporter moves over theobstacle, however, the increased torque will accelerate the wheels sincethe obstacle no longer resists the wheels. The wheel acceleration willcause the wheels to move ahead of the transporter's center of gravity(CG) and cause the transporter to pitch backward. The balancingcontroller will detect the pitching condition through either a change inpitch error or through a change in the transporter velocity and commanda decrease in the current supplied to the ideal motor thereby reducingthe torque applied to the wheel.

If the delay caused by the balancing controller is negligible and theaccuracy of the velocity information fed back to the balancingcontroller is extremely high, the rider will not notice a differencewhether voltage or current control is used. However, if the controlleror shaft sensor selected for the transporter has a limited bandwidth,current control mode will not provide the prompt response that voltagecontrol mode exhibits for small obstacles. In a preferred embodiment ofthe invention, a low-cost Hall effect sensor is employed to detect shaftrotation. In addition, for reasons described below, limitations on theselection of the gains used in the control law for current control moderesult in a softer transporter response relative to voltage controlmode.

Rider Detector

Operating modes of the transporter may include modes wherein the rideris supported by the transporter but may also include modes where therider is not supported by the transporter. For example, it may beadvantageous for the rider to be able to ‘drive’ the transporter whilewalking alongside or behind it.

Additionally, it is advantageous for certain safety features of thetransporter to be triggered if the rider leaves the transporter whilethe transporter is in motion. FIGS. 6a and 6 b show a rider detectionmechanism used in an embodiment of the present invention. FIG. 5a showsa top view of the rider detector designated generally by numeral 510.Transporter 10 incorporating the rider detector includes a base 12, leftwheel fender 512, right wheel fender 514, support stem 16 for handlebar14 (shown in FIG. 1). Wheel fenders 512 and 514 cover the correspondingwheels. Support stem 16 is attached to the base 12 and provides a sealedconduit for transmission of signals from controls 32, 34 (shown inFIG. 1) that may be located on the handlebar to the control electronicssealed in the base 12. Wheel fenders 512, 514 are rigidly attached tothe sides of the base.

The top of base 12 provides a substantially flat surface and is sized tocomfortably support a rider standing on the base 12. A mat 521 coversthe top of the base 12 and provides additional protection to the base 12from particles and dust from the environment. In an alternateembodiment, the mat may also cover part of the fenders 512, 514 and maybe used to cover a charger port (not shown) that provides for externalcharging of the power supply. Mat 521 may be made of an elastomericmaterial that provides sufficient traction such that the rider does notslip off the mat 521 under expected operating conditions. A plate 522 ispositioned between base 12 and mat 521. Plate 522 is made of a rigidmaterial and evenly distributes the force acting on the plate 522 fromthe rider's feet such that at least one rider detection switch 523 isactivated when a rider is standing on the mat.

FIG. 6b shows a cut side view of rider detector 510. Switch 523 is madeof an elastomeric material that may be fabricated as an integral part ofthe base cover 524. Although the fabrication cost may be greater, makingthe switch 523 integral with the base cover 524 eliminates a possibleleak source. Switch 523 has a stem 540 extending below base cover 524and a top 542 that extends above the base cover 524. When top 542 isdepressed, switch 523 deforms such that a stem 540 is displaced downwardtoward an electronics board 550 that is sealed within base 520. Anoptical switch is located on the electronics board 550 such that whenstem 540 is displaced downward, stem 540 interrupts a light beam 557generated by a source 555 and the light beam interruption is detected byan optical detector 556.

The mat edge 525 is preferably attached to the top of the base cover524. Mat 521 has a raised portion 527 that is support by a wall 526connecting the mat edge 525 to the raised portion 527. The height of thewall 526 is sized such that plate 522 does not exert a force on theswitch 523 when there is no weight on the mat 521. When the rider stepson the raised portion 527, plate 522 is displaced toward electronicsboard 550 until stem 540 interrupts light beam 557. When the rider stepsoff of the transporter, mat 521 returns to the raised configuration asdoes switch 523 thereby re-establishing light beam contact between thesource 555 and detector 556.

Steering Device

Referring now to FIG. 7, an exploded view is shown of an embodiment of asteering device for a scooter-like vehicle such as the balancing vehicle10 of FIG. 1. A potentiometer 602, or other sensor of the position of arotatable shaft 604, is attached to a housing 606. The housing may bepart of handlebar 14 (shown in FIG. 1). A rotatable grip 608 is attachedto potentiometer shaft 604 and provides a grip for the rider. Atorsional spring 610 is connected at one end to the rotatable grip 608and at the other end to the potentiometer 602 or to housing 606. As therider rotates grip 608, the grip turns shaft 604. Potentiometer 602,with voltage suitably applied across it, as known in the art, generatesa signal substantially proportional to the rotation of the shaft. If therider releases the grip, torsional spring 610 rotates grip 608 and theshaft to their respective neutral or zero positions. Return of grip 608to its neutral position allows the transporter to continue traveling inthe same direction as when the grip was released. If the grip was notreturned to the neutral position when released, the transporter wouldcontinue to turn in the direction of the residual rotation.

The direction of rotation may be used to encourage the rider to leaninto the turn. For example, referring further to FIG. 7, if the rider'sright hand holds grip 608, a twist in the direction of the rider'sfingers corresponds to a right turn. The rotation of the rider's rightwrist to the outside of the handlebar encourages the rider to shiftweight to the right and into the turn. Shifting weight into the turnimproves the transporter's lateral stability.

Referring now to FIGS. 8a-8 d, a thumb-activated, elastomer-damped,steering input device is shown and designated generally by numeral 620.A rotation sensor 622, which is preferably a potentiometer but may beany rotation sensor, is coupled to a structure fixed, with respect torotation, to the support of a personal transporter, preferably tohandlebar 14 (shown in FIG. 1). A shaft 624 of the steering device 620is bent with respect to a pivot point 626 in response to force appliedto thumb button 630 by thumb 628 of the user. As shaft 624 is bent,local rotation about pivot 626 is read by rotation sensor 622, and asignal characteristic of the rotation is transmitted to the transportercontroller. Shaft 624 of input device 620 is comprised of elastomericcore 632 surrounded by metal sheath 634. Elastomeric core 632 may berubber, for example. Distal end 636 of shaft 624 is captured betweenlimit posts 638 which extend from the handlebar and which limitdisplacement of shaft 624 when the user rotates the proximal end 640 ofthe device.

User's rotation of proximal end 640 causes shaft 624 to bend as shown inFIG. 8b. Metal sheath 634 acts as a leaf spring, providing a restoringforce that counters user's rotation of the device, and brings the deviceback to the neutral configuration depicted in FIG. 8a. Elastomeric core632 acts as a shear spring that opposes rotation of the device by theuser and increases the opposition as the deflection increases. Increasedopposition arises due to differential sliding between metal sheath 634and elastomeric core 632 as the long (distal) end 624 is bent. The backview of steering input device 620 shown in FIG. 8c shows potentiometer622 for generating a signal substantially proportional to rotation ofshaft 624. The top view of steering input device 620 shown in FIG. 8dshows the roughly L-shaped elbow 642 of the proximal end 640 of inputdevice 620. Dashed outline 644 depicts the steering input device in thedeflected condition corresponding to FIG. 8b.

A further steering device for the personal transporter 10 of FIG. 1 isshown in FIGS. 9a and 9 b, in accordance with another embodiment of theinvention. Palm steering device 650 is contained on the surface ofhandlebar 14. In the rest state depicted in FIG. 9a, upper surface 652of lever 652 is substantially parallel to and substantially flush withupper surface 656 of handlebar 14. Lever 652 is constrained to rotateabout pivot 658 which is substantially parallel to the ground andparallel to the forward direction of motion of the transporter. Therider places a palm of a hand over lever 652 and, by pressing one side660 or the other of lever 652 about pivot 658, causes generation of asteering signal. The steering signal is generated by a rotation sensor662 at the pivot 658 or by pressure sensors either side of fulcrum 664.

Inertial Measurement Unit

The inertial measurement unit (IMU) houses the sensors used by theprocessor to determine the orientation and speed of the transporter.Full redundancy may be accomplished through the use of two IMUs that arepreferably physically separated from each other and powered by separatepower supplies as shown in FIG. 2. Spatial constraints may require theredundant IMUs to be housed in the same package while still maintainingindependent power supplies and independent signal lines to separateprocessors.

In an embodiment of the present invention, the A-side and B-side IMUs181 and 182 (shown in FIG. 2) are housed in a single package. Each IMUmay be equipped to measure the transporter orientation about three axes(pitch, yaw, and roll), about two axes, or about one axis (pitch). Inanother embodiment, each of the A-side and B-side IMUs is equipped tomeasure the transporter orientation about three axes. In anotherembodiment, a three-axis IMU may be paired with a single axis IMU.

Each IMU includes a sensor 190 (shown in FIG. 2) and the supportingelectronics for the sensor. The sensor may be any device capable ofgenerating a signal that is indicative of the orientation or the rate ofchange of orientation of the sensor. The generated signal is preferablynearly proportional to the orientation or rate of change of theorientation of the sensor, but other dependencies are within the scopeof the present invention. For example, a sensor may be a liquid levelpendulous tilt sensor, a physical gyroscope, a solid-state gyroscope, anaccelerometer, or a pair of proximity sensors arranged in a line andseparated by a known distance. In various embodiments of the presentinvention, a solid-state gyroscope is used with a liquid level tiltsensor. The liquid level tilt sensor may be used to correct for drift inthe solid-state gyroscope as described in U.S. application Ser. No.09/458,148 herein incorporated by reference.

A single axis IMU may consist of a solid-state gyroscope and a tiltsensor with both sensors mounted to provide a signal corresponding tothe pitch orientation of the transporter. The 3-axis IMU consists of atleast three solid-state gyroscopes and a tilt sensor. The gyroscopes maybe mounted to provide signals that correspond to a mixture of any of therotations about three mutually orthogonal axes. Alternatively, thegyroscopes may also be mounted to avoid saturation of the gyroscopesignal. The orientation of the gyroscopes will depend on the spaceconstraints of the IMU housing, the saturation limits of the gyroscopes,and the expected performance requirements of the transporter. In oneembodiment of the present invention, the 3-axis IMU consists of foursolid-state gyroscopes and a tilt sensor. Use of four gyros enables theIMU to detect a failure in one of the gyros. Although the identity ofthe failed gyro cannot be determined, the existence of a failure issufficient to alert the processor to take the appropriate action, asdescribed below, while maintaining rider safety and comfort.

Processor

In various embodiments of the present invention, a control programrunning on a processor determines the dynamic state of the transporterand calculates the appropriate command to send to the motor amplifiercontrollers based on the dynamic state of the transporter and on anyrider commands. In a preferred embodiment, the processor also calculatesthe appropriate switch commands to the power stage 324 (shown in FIG. 4)thereby eliminating the need for a separate amplifier controller. Theprocessor may be a digital signal processor (DSP) optimized forcontrolling motors. The term ‘processor,’ as used herein, alsoencompasses within its scope an embodiment in analog circuitry of thefunctions described. The circuitry and associated electronic componentsrequired to support the processor are well known in the electroniccontrol circuit art.

Referring now to FIG. 10, a logical flow diagram is presented of thecontrol program executed by the processor. When the rider activates thetransporter, the control program performs an initialization procedure705. The initialization procedure performs redundancy checks between theprocessors, checks for any subsystem faults, and initializes the IMUs.After the subsystems and processors have passed the initializationchecks and the IMUs are initialized, the initialization procedure alertsthe rider that the transporter is ready for use. The alert may be anaudio or visual indicator such as a tone or a light. In a preferredembodiment, the initialization procedure gives the ready alert to therider after the 1-axis state estimator has initialized. This allows therider to begin using the transporter while the 3-axis state estimator isstill initializing.

The program next checks for rider commands and transporter state sensorsignals in 710. The rider commands may include rider detection describedabove, yaw commands, pitch trim commands, emergency brake commands, andmode change commands. The transporter state sensor signals may includesensors for measuring the temperature of the transporter components suchas battery or motor temperature or potential sensors for measuring thevoltage of the battery pack. The state sensors also include the sensorsin the IMUs.

The program in 715 determines the transporter orientation based on thesensor signals from the IMUs. In a preferred embodiment, a 3-axis IMUincorporating four solid state gyros and a two-axis tilt sensor,designated as the A-side IMU, is paired with a 1-axis IMU, designated asthe B-side IMU.

The program first checks for a gyro failure in the A-side IMU bycomparing the combined signals from two subsets of the four gyros. Ifthe program determines that one of the four gyros has failed, theprogram sets an A-side IMU fault flag that will activate a procedure tobring the transporter to a safe condition as described below. Theprogram also estimates the transporter orientation based on the signalsfrom the B-side IMU. If the A-side IMU is not faulted, the B-sideestimate is compared to the A-side estimate. If the B-side estimatediffers from the A-side estimate by more than a preset amount, theprogram sets a B-side IMU fault flag which will also activate the safecondition procedure. If the B-side estimate agrees with the A-sideestimate to within the same preset amount, the program disregards theB-side estimate and uses the A-side estimate for further processing withthe knowledge that the B-side IMU is available to safely bring thetransporter to a stop should the A-side IMU fail.

In another embodiment of the present invention, both the A-side andB-side IMUs are 1-axis state estimators.

The program generates the wheel motor commands in 720. This portion ofthe program is also referred to as the balance controller. The balancecontroller is described in U.S. Pat. No. 5,971,091 and U.S. applicationSer. No. 09/458,148, both of which are hereby incorporated by reference.

The wheel motor commands are generated through a control law having theform

 Command=K₁θ+K₂θ_(r)+K₃x+K₄x_(r)

where

θ=transporter pitch error

θ_(r)=transporter pitch rate error

x=transporter position error

x_(r)=transporter velocity error

The dynamic state variables are in the form of an error term defined asthe desired value minus the measured value. For example, θ is thedesired transporter pitch minus the measured transporter pitch. Themeasured transporter pitch and pitch rate are determined from the IMUsignals. The measured transporter position and transporter velocity aredetermined from the shaft feedback sensors. For balanced operation, thedesired pitch rate is set to zero. The desired pitch may be adjusted bythe rider through a pitch trim control and may also be adjusted by thecontrol program during transporter operation.

The adjustable coefficients, K₁, K₂, K₃, and K₄, are commonly referredto as gains and together form a set of coefficients that define anoperating mode. As the values of the coefficients change, theresponsiveness and stability of the transporter changes. The gains areset to a value as specified by the user in selection of a mode ofoperation of the vehicle. For example, K₃ is normally set to zero toallow the transporter to travel but K₃ can be set to a positive value toenable the transporter to remain balanced at a stationary point.

In one embodiment, K₁ is set to a positive value and K₂, K₃, and K₄ areset to zero. In this operating mode, the transporter does notautomatically balance but the rider may maintain balance and commandfore/ aft motion of the transporter by adjusting his/her weight in thefore/aft direction while traveling. Unlike a motorized scooter ormotorcycle where the rider maintains lateral stability while commandingfore-aft motion, the transporter of the present invention operating withonly a non-zero K₁ requires the rider to maintain balance in thefore-aft direction while simultaneously commanding fore-aft movement.The higher level of skill required to operate the transporter in such amode may be appealing to some riders for its recreational value.

In another embodiment, K₁ and K₂ are set to positive non-zero values andK₃ and K₄ are set to zero. In this mode, the transporter is capable ofmaintaining balance and requires a steady-state ‘error’ (or ‘offset’) inpitch in order to maintain a steady-state speed. However, a rider coulddevelop the skill to operate the transporter in a balanced state whileavoiding instabilities through proper control of the rider's weightshifting.

In typical operation, only K₃ is set to zero. In this mode, thetransporter maintains a small pitch ‘error’ while traveling at a steadyspeed. The responsiveness of the transporter may be modified byadjusting the values of each of the gains relative to each other. Forexample, if K₁ is increased, the rider will perceive a stiffer responsein that a small lean in the forward direction will result in a largewheel command for traveling forward over bumps or accelerating rapidlyin the forward direction. However, the gains cannot be adjusted in acompletely independent manner and still have the transporter remainstable. The bandwidth of the sensor signals (velocity, pitch, pitchrate, etc.) as well as the bandwidth of the actuator (transmissionstiffness, torque bandwidth) place an upper limit on the achievablestiffness. For another example, if the shaft feedback sensor is capableof providing a high resolution velocity signal with very small delay andthe processor is capable of a high frame rate, the gains may beincreased to provide a stiff transporter response while avoidingoscillatory instability. Conversely, if the shaft feedback sensorgenerates a noisy velocity signal or the processor frame rate is onlymoderate, the ability to increase the gains will be limited and therider will experience a “mushy” or “sloppy” transporter response.

The motor commands generated by each of the A- and B-processors 135, 145(shown in FIG. 2) are compared in step 725 of FIG. 10. If the commandsdiffer by more than a preset amount, a processor fault flag is set thatwill activate a safe shutdown routine for the transporter. If the motorcommands are within the preset amount of each other, the commands areaveraged and the averaged command is sent to the motor amplifiercontrollers in step 730. The program checks an internal clock in 735 andtransfers execution to 710 at the appropriate time. The program loop710, 715, 720, 725, 730, 735 is referred to as a frame and is executedat least 5 times per second and preferably at least 100 times persecond. Frame execution rates below 100 Hz may appear to the rider as anunsteady or unstable transporter. Higher frame rates increase thesteadiness of the transporter as perceived by the rider.

Closed Loop Yaw Control with Position

Steering, or yaw control, of the transporter is accomplished by adding aturning command to the wheel amplifiers and have the following form.

LeftCmd=BalCmd+YawCmd  (1)

RightCmd=BalCmd−YawCmd  (2)

The LeftCmd and RightCmd are the command sent by the controller to theleft and right motor amplifiers, respectively. The LeftCmd and RightCmdrepresents voltage if the amplifiers are in voltage control mode,current if the amplifiers are in current control mode, or duty cycle ifthe amplifiers are in duty cycle control mode. BalCmd is the commandsent by the controller to each amplifier to maintain the transporter ina balanced state while moving or while at rest. The YawCmd causes thetransporter to turn by reducing the command to one of the wheels whileincreasing the command to the other wheel. For example, a positiveYawCmd increases the command to the left wheel while decreasing thecommand to the right wheel thereby causing the transporter to execute aright turn. The YawCmd may be generated by a yaw-input device describedabove with no feedback loop.

In addition to steering the transporter, the yaw controller should alsobe relatively insensitive to transient yaw disturbances. An example of ayaw disturbance is when one of the wheels travels over a small obstacleor bump. The wheel encountering the obstacle will slow while the otherwheel continues at the original velocity thereby turning the transporterin the direction of the obstacle. A sudden, uncommanded change in thedirection of travel is undesirable in any transportation device. In apreferred embodiment, a closed loop yaw controller is implementedfollowing a control law given by:

YawCmd=k_(p)ψ_(error)+k_(d)ψ′_(error)  (3)

where ψ_(error) is given by (ψ_(desired)−ψ), ψ′_(error) is given by(ψ′_(desired) −ψ′), ψ′ is the yaw rate given by ψ′=c·(ω_(R)−ω_(L)), ψ isthe yaw given by ψ=∫ψ′dt, k_(p), k_(d), and c are constants and ω_(R)and ω_(L) are the right and left wheel angular velocities, respectively.The desired yaw rate, ψ′_(desired), and desired yaw, ψ_(desired), may beprovided by the controller or by the rider. The transporter may be maderelatively insensitive to yaw disturbances by selecting a large valuefor k_(d). If k_(d) is large, a small yaw rate error will produce alarge YawCmd that will act to counter any disturbance-induced turning ofthe transporter. However, if k_(d) is too large, the transporter will besusceptible to yaw instabilities that depend, in part, on the mechanicalproperties of the wheels and on the coupling behavior between the leftand right wheel.

The gain, k_(p) is used to correct yaw position errors. Depending on theactuator drive method (current mode, voltage mode, or duty cycle mode),k_(p) will be more or less important in reducing the yaw error that isintroduced by a disturbance force.

In one embodiment, the yaw control law for the left and right wheels ismodified to replace the yaw rate dynamic variable ψ′=c·(ω_(R)−ω_(L))with the left and right wheel angular velocities, ω_(R) or ω_(L),respectively. Using the wheel velocities instead of the yaw rate in theyaw control law removes the coupling between the left and right wheelthereby allowing the damping gain, k_(d), to be set to a higher valuefor a stiffer yaw control. However, the mechanical properties of thewheels place an upper limit on k_(d) and therefore limit the yawstiffness of the transporter.

As discussed above, motor amplifiers 132, 133, 142, 143 are preferablyoperated in voltage control mode. As discussed, voltage control allowsthe motor to provide an almost instantaneous feedback loop to maintainthe wheel velocity during transient events due to the back-emf of themotor. The effect of controlling voltage is that a term proportional tok²ω/R is added to the yaw control law where k and R are characteristicof the motor, as described with reference to FIG. 5, and (ω is the rightor left wheel velocity for the right or left yaw control law,respectively.

Traction Control

As discussed above, the controller maintains the transporter in adynamically balanced condition by commanding either the wheel torque orwheel speed, hereinafter referred to inclusively as wheel torque,through the power amplifiers and wheel motors. The controller monitorsthe orientation of the transporter through the inertial reference deviceand adjusts the wheel torque to maintain balance. The coupling betweenthe wheel command and transporter orientation will depend on, interalia, the traction between the wheel and the underlying surfacehereafter referred to as the ground. If the commanded torque to thewheel exceeds the frictional breakaway force between the wheel and theground, the wheel will slip and adversely affect the controller'sability to maintain the transporter in a balanced state. A first type ofloss of traction, referred to herein as a “Type I” loss of traction, mayoccur if the rider tries to accelerate (or decelerate) faster than thelocal condition of the wheel and the ground allow. A second type of lossof traction, herein “Type II”, may occur when the transporter encountersa slick spot, such as black ice, on the ground or when the transporterloses contact with the ground such as when driving the transporter offof a ramp. In both types of loss of traction, the wheel will accelerateas the wheel slips.

Referring now to FIG. 11, a flow diagram is shown of a method oftraction control in accordance with embodiments of the presentinvention. The controller continuously monitors the wheel speed andcalculates a wheel acceleration, A_(W), in 810. The controller alsoestimates the inverse wheel inertia, J_(W), in 820 by dividing the wheelacceleration from the previous step 810 by the commanded torque providedfrom the balancing routine described above. The inventors havediscovered that the inverse wheel inertia is a wheel characteristiccapable of distinguishing between a Type I loss of traction and a TypeII loss of traction. The calculated wheel acceleration is compared to apre-set value, A_(MAX), in 830. The pre-set value corresponds to anacceleration characteristic of a loss of traction and depends on thetransporter characteristics in a manner readily determinable. If A_(W)is greater than A_(MAX), the controller sets a flag indicating a slipcondition in 835. The controller checks the flag in 840 and if the flagis not set, the controller executes a torque slewing routine 870described below. If the flag is set, indicating a slip condition, thecontroller allows the slipping wheel to free wheel. This may beaccomplished by disabling the motor amplifiers commanding the slippingwheel. In a preferred embodiment of the present invention, thecontroller sets a torque offset to the negative of the torque command in850 such that the sum of the torque offset and torque command that issent to the motor amplifiers is zero, thereby allowing the wheel torotate freely and reduce the acceleration of the wheel. In anotherembodiment of the present invention, the torque offset is set to a valuesuch that the sum of the torque offset and torque command is less thanthe torque command.

The controller compares inverse wheel inertia J_(W) to a pre-set value,J_(min) in 860. If J_(W) is less than J_(min), the controller clears theslip flag in 865. The inverse wheel inertia term is used to clear theslip condition because it can distinguish between a Type I and Type IIloss of traction. For example, if the wheel loses contact with thesurface, J_(W) will be very large because the moment of inertia willonly include the wheel and will be small. Conversely, J_(W) will besmall when the wheel remains in contact with the ground because themoment of inertia will include the whole transporter and will be large.

The torque offset is decremented or slewed to zero in 870. This allowsfor a smoother transition for the rider after the transporter recoversfrom the slip condition.

Deceleration to Zero

As discussed previously, the rider may control the fore/aft movement ofthe transporter by leaning. However, situations may arise where thetransporter must be brought safely to a stop before the rider can reactto the situation. For example, if a component used by the balancingcontroller fails, the controller may not be able to maintain the movingtransporter in a dynamically balanced condition. If a component failureis detected, a deceleration-to-zero routine is executed by thecontroller to automatically bring the transporter to a stop, therebyallowing the rider to dismount from the transporter before thecontroller loses the capability to maintain dynamic balancing.

In FIG. 12, a flow diagram is shown for the deceleration-to-zeroroutine. The routine is entered in 910 every controller frame. If acritical fault is not detected in 920, the routine is exited in 930. Acritical fault may be any one of a variety of conditions that couldaffect the balancing controller. For example, a battery open condition,CPU/RAM failure, motor winding open condition, motor winding shortcondition, or tilt sensor failure indicate that redundancy has been lostfor that component and the controller should bring the transporter torest. Other faults, such as battery over-temperature or motorover-temperature may indicate an imminent failure of the component andmay also be used to initiate the routine to decelerate the transporterto rest.

The transporter has a function which limits the speed of travel and isdescribed in U.S. Pat. No. 5,791,425, which is herein incorporated byreference. If a critical fault is detected in 930, the controller bringsthe transporter to a stop by slewing the speed limit at the time thefault is detected to zero in 940. It should be understood by one ofordinary skill in the art that slewing is the process of incrementallychanging a variable from an original value to a final value over severalcontroller frames. Since each controller frame corresponds to a timeinterval, the number of frames over which the slewing process iscompleted corresponds to the time it takes to bring the transporter to astop. The stopping time will depend on several transporter dependentfactors and on rider comfort. For example, if the transporter is broughtto a sudden stop, the rider may feel discomfort at the sudden andunexpected stop. Conversely, if the stopping time is very long, theprobability that the backup component may fail increases. In anotherexample, the stopping time may be based on the properties of thespecific sensors used in the transporter. In one embodiment of theinvention, a tilt sensor is used to correct the drift of a gyroscope. Ifthe tilt sensor fails, the information provided by the gyroscope mayremain adequate for the balancing controller until the gyroscope driftcreates an error that adversely affects the controller's ability tomaintain the dynamically balanced condition of the transporter. If thegyroscope has a low drift rate, the information will remain adequate fora longer period and a relatively larger stopping time may be used.Conversely, if the gyroscope has a high drift rate, a smaller stoppingtime will be required. In an embodiment of the present invention, thestopping time is between 1 and 10 seconds, preferably between 2 and 4seconds.

Depending on the particular fault condition, the deceleration-to-zeroroutine may also adjust the commands to the remaining, non-faultedcomponents to compensate for the failed component. For example, if amotor winding fails, the motor will continue to operate but at only halfthe power. A sudden reduction in one of the motors would cause thetransporter to suddenly turn. To prevent such a sudden turn, the commandto the operating winding of the motor is doubled to compensate for thefailed winding. However, doubling the command to the remaining motorwinding may exceed the operating limits on the motor amplifier causingthe amplifier to fail. The expected period that the motor amplifier canfunction over its operating limits may determine the stopping time.

The transporter does not require a brake, in the sense of having adevice for applying an external opposite torque to the wheel, becausethe controller and motor amplifier controls the position of the wheeldirectly. As mentioned previously, the fore-aft motion of thetransporter is controlled by the leaning of the rider so if the riderwishes to stop, the rider merely leans in the direction opposite to thedirection of the moving transporter. Although the rider's actions arethe natural motions that a walking person would make, riders accustomedto operating powered vehicles may expect to use a brake to slow thetransporter and in an unexpected situation may instinctively reach for abrake instead of merely leaning backward.

In one embodiment of the present invention, a brake control isincorporated into the handlebar controls. The brake control may be asimple two-state device such as an on-off switch or the switch may be aproportional device generating a signal proportion to the rider input.Activation of the brake switch causes the controller to execute thedeceleration-to-zero routine described above with the followingmodification. The “fault condition” in 920 is the activation of thebrake switch. Since the fault in this case is not a component used bythe balancing controller, the stopping time (number of processor framesfor the stewing process) may be lengthened to a more comfortable ratefor the rider. In one embodiment, the stopping time is between 5 and 10seconds.

If the brake control is a proportional device such as a pressure sensor,the rate of deceleration may be controlled by the rider through theapplication of pressure on the brake control. If the rider applies ahigh pressure, the rate of deceleration is increased by decreasing thenumber of slewing frames. Conversely, if the applied pressure is low,the deceleration rate is lowered by increasing the number of slewingframes.

What is claimed is:
 1. A method for maintaining traction between wheelsof a transporter and an underlying surface, the method comprising: (a)comparing acceleration of a wheel with a specified pre-set value; (b)setting a slip condition flag based on the acceleration of the wheel;(c) reducing the torque applied to the wheel based on the slip conditionflag; (d) determining a value of a dynamic characteristic of the wheel;and (e) clearing the slip condition flag based on the value of thedynamic characteristic of the wheel.
 2. The method of claim 1, whereinthe dynamic characteristic is a moment of inertia.
 3. The method ofclaim 1, wherein the dynamic characteristic is an inverse of a moment ofinertia.
 4. The method of claim 1, wherein the step of determining thedynamic characteristic includes dividing the acceleration by a commandedtorque applied to the wheel.
 5. The method of claim 1, wherein the stepof reducing the torque applied to the wheel includes reducing the torqueto zero.
 6. The method of claim 1, wherein the step of reducing thetorque applied to the wheel includes slewing the torque incrementally.7. A device for correcting wheel slippage on a vehicle, the devicecomprising: (a) a sensor for monitoring wheel speed; (b) adifferentiator for calculating wheel acceleration based on change inwheel speed; (c) a comparator for comparing the wheel acceleration witha pre-set value and for setting a slip condition flag; and (d) acontroller for reducing any torque applied to the wheel, such thattorque continues to be reduced until the slip condition flag is cleared.